In the semiconductor industry, intricate designs or patterns of electronic chips are generally made using lithographic techniques, such as photolithography, X-ray lithography, or extreme ultraviolet (EUV) lithography. These techniques utilize a patterned photomask or reticle in combination with certain systems to transfer patterns onto objects such as semiconductor wafers and electronic chips. For example, in a photolithographic process, a patterned photomask is used in combination with laser exposure systems to transfer patterns. Processing situations, however, may distort the resulting pattern defined on a semiconductor wafer. For example, optical diffraction may cause the pattern defined on the wafer to differ from the pattern of the photomask.
A photomask may include assist or auxiliary features that compensate for distortions in a resulting pattern transferred onto a wafer. The auxiliary features aid in the transfer of primary features of the photomask. In one technique for compensating distortions, a photomask may include sub-resolution assist features (SRAFs). An SRAF is designed to improve the process margin of a resulting wafer pattern, but not to be printed on the wafer. Typically, the SRAF is small enough and properly located on the mask so that that the SRAF is not transferred onto the wafer because the wafer features are below the dimensional resolution of the lithography system. The SRAF, however, is large enough to affect the passage of light and impacts a nearby lithographic feature.
In certain situations, however, the SRAFs may be unsatisfactory. For example, the SRAFs may print on a wafer or may violate mask rules. The unsatisfactory SRAF may be caused by interactions between SRAFs of neighboring printed photomask features or neighboring SRAFs. Accordingly, the position of the SRAFs must be accurately determined in order to prevent unsatisfactory effects in the photomask. Typically, the SRAFs positions are determining by simulating the entire mask layout including all primary and secondary features and looking for any interactions.
FIGS. 1A and 1B are diagrams illustrating a conventional method for determining the interaction of mask features for positioning SRAFs. As illustrated in FIG. 1A, a mask design 100 includes several wafer features, such as contact holes 102. Contact holes 102 are positioned in the design according to the requirements of the design.
FIG. 1B illustrates a conventional method for determining the interaction of contact holes 102 in mask layout 100. As illustrated in FIG. 1B, projections 104 are simulated for each contact hole 102. If projections 104 overlap, the overlapping edge is considered to interact with the corresponding overlapping edge. As such, the SRAFs' position and number must be determined considering the interaction. The SRAFs' position and number is determined by the pitch between the interacting contact holes. If projections 104 do not overlap, the non-overlapping edges are considered to be isolated (ISO) edges 106. As such, the conventional method will not consider the interaction between ISO edges 108 even though these contact holes are in close proximity.
According to the conventional method, the entire design must be simulated first to determine if the lithographic features and SRAFs will interact. Further, the conventional method does not recognize all possible interactions between lithographic features. In FIG. 1B, according to the conventional method, edges 108 of contact 102 do not interact. Due to their proximity, however, contacts 102 with edges 108 are not independent, but are strongly coupled. As such, if the conventional method was utilized, the SRAFs of these contacts 102 would interact and may print on the wafer. Alternatively, if the conventional method was utilized, the process margin of the main feature may be insufficient to meet the requirements of the process